In recent years, industry has developed efficient methods for producing molded articles through reaction injection molding (RIM). Typically, plastic slugs are forced under extremely high pressures at very high speeds into molds in order to form the plastic into the shape of the desired article. High temperatures usually accompany such processes.
RIM articles may be created very quickly and are generally well suited for high volume production. The molded article can cure in the mold in as short a period of time as 90 seconds. Once the article is cured, the mold is opened or separated, the article removed, and the mold closed for the molding of the next piece. Thus, such a single cavity mold could produce as many as 40 pieces per hour. A multi cavity mold could produce proportionally more pieces per hour.
A major drawback with RIM articles is that excess unwanted material is molded adjacent the article. This is known as flashing and occurs at joints in the mold, at the split line between two halves, at the inlet for the material, or across open cavities formed in the article. There are numerous locations and causes of flashing. Flashing almost always forms around the entire perimeter of a molded article.
Deflashing is the process of removing the unwanted flashing from the article. This process is very labor intensive because of the length of time it takes and the inability to automate it. For rigid materials which, for the purposes of this patent, includes semi-rigid materials such as plastic, the flashing is typically cut by hand held knives.
The workpiece is usually fixtured in place and then a person uses a sharp knife, either manual or pneumatic, to cut away the flashing from the perimeter or edges of the workpiece. Once the flashing has been cut away, the process of deflashing is completed by smoothing the newly exposed edges with emery cloth or other slightly abrasive material.
The process of deflashing can take eight minutes or longer, depending upon the complexities of the workpiece. If the RIM process takes 90 seconds, the total time per piece is 9.5 minutes. To keep up with high volume capacity of a single cavity mold, six people would be required in this example. Thus, it is clear that a need exists to more efficiently deflash such workpieces.
Several methods have been tried without success. Automatically guided knives designed to trace the outline of the flashing based on computer control or tracing has not been successful. To practice this method, it is necessary to fix the workpiece precisely within a few thousandths of a theoretical position each time. Such fixing has not economically been achievable. Furthermore, the workpiece is subject to flexing and displacement when the cutting force of a knife is exerted against the material. Not only does this affect the accuracy of automated cutting (which is unacceptable), but it also limits the practical speed at which the flashing may be cut off.
Deflashing has also been attempted through abrasive particle blasting, such as sand blasting. This process has not met with success due to the damage caused to the exterior surface of the article. As would be expected, the surface finish of the manufactured article is very important while sand blasting can remove the flashing, it devastates the surface and is unacceptable.
There is a need for a process which can quickly deflash a workpiece without affecting the surface finish nor other physical characteristics of the workpiece.